This invention relates to electronic calculators having digital readouts, wherein said readouts are in the form of a display cell incorporating an electro-optically passive material such as liquid crystal material. Specifically, this invention relates to such electronic calculators incorporating large-scale integrated circuits for driving the display cell and performing calculating functions.
In recent years, the use of display cells incorporating passive elements, as opposed to electro-optically emissive elements such as light-emitting diodes, has rapidly increased in timepieces, calculators, measuring apparatus and the like. Such increased use results from the low power consumption characteristic of such passive elements. Passive display elements are characterized by the selective rendering of regions of the display cell visually distinguishable from adjacent regions. The selected regions are generally shaped as a character (based, by way of example, on a seven-bar display for numerics). A number of approaches can be used to render the selected region visually distinguishable, including selectively rendering regions of the passive element light-transparent, with the region being rendered visible through reflection or light transmission. Electric fields or current applied to the passive elements can change the optical light rotary-power characteristic of a region to render the region transparent or can cause light-scattering in the region by applying an oscillatory motion such as the Brownian movement to the crystal. Passive elements, as used herein, are not limited to liquid crystal materials but include other passive materials, such as, by way of example, substances which apply the double refractivity caused by the electric field of a ferroelectric (Pockels Effect or Kerr Effect). By way of example, the calculator depicted herein incorporates a liquid crystal display cell and is adapted for use where the supply voltage is low, namely, 1.5 volts to 30 volts.
Specifically with regard to liquid crystal materials, there are two types of driving characteristic, namely, the Field Effect Mode (FEM), wherein the orientation of the liquid crystal molecules is controlled in order to obtain the rotary-power optical activity by the field effect, and the Dynamic Scattering Mode (DSM), wherein the complicated motion of the liquid crystal molecules is caused by the applied current. FEM liquid crystal materials have extremely excellent low-power consumption characteristics on the order of 10.mu.W or less in the case of a digit display of about eight figures. Such liquid crystal displays produce a wristwatch which can be driven by a battery and which is characterized by a life of one year or more.
However, liquid crystal materials do not have the diode characteristic of a display cell made of active elements such as light-emitting diodes and fluorescent display tubes, so that the duty factor for dynamic driving is large and many controlling wires are required. The duty factor of a passive element is a quarter to at most one-tenth of the duty factor of an active element. For this reason, the application of liquid crystal display cells to calculators causes an economic loss because the input and output terminals of the large-scale integrated circuit which contains the bulk of the calculator circuitry increase in number.
This results in an increase in the area of the silicon wafer defining the large-scale integrated circuit, modification of standard tools for assembling such large-scale integrated circuits and increased inspection time, thereby resulting in increased costs of production. In other words, the marked advantages of liquid crystal material are nullified by the increased costs necessitated by the large-scale integrated circuits of increased size. This produces the anomalous result that while a liquid crystal display for a calculator enables the provision of a battery power source having a long life, the resulting calculator is inferior to a calculator using active elements because of the price thereof. By using the display-driving lines as input lines for the keyboard, the foreging deficiencies are avoided.